1. Field of the Invention
The apparatus of the present invention relates to oil and gas drilling. More particularly, the present invention relates to an apparatus placed in the drill string above the drill bit for allowing intermittent reduced hydrostatic pressure at the drill bit in order to increase drill rate yet maintain normal and adequate hydrostatic pressure throughout the well bore.
2. General Background
In the drilling of oil and gas wells, the most common method of drilling is the positioning of a drill bit at the lowermost end of a continuous drill string whereby rotation of the drill string imparts rotation to the bit and the bit bores a continuous hole down to the source of hydrocarbons. During the drilling process, it is imperative that the cuttings, etc. at the face of the bit that are being drilled out be washed away from the bit so that the bit may achieve maximum drilling during the process.
Current state of the art drilling operations utilize high velocity streams directed at the bottom of the well bore to facilitate cutting removal. This is accommplished by the downflow of fluids through the bore of the continuous drill string which are ejected at the bit in order to wash cuttings away and the washed away cuttings and fluid move upward along the annular space between the drill string and the casing wall or the surrounding earth.
One of the problems confronted in achieving this drilling method is the fact that the hydrostatic pressure of the fluid exerts a force on the strata below the bit and reduces significantly the drill bit's ability to drill up to its peak efficiency, and thus, of course, increases the amount of time in order to drill the bore hole. Of course this increase in time causes a delay in drilling activity and increased expenses in the drilling of the borehole.
Several apparatuses have been developed and patented which attempt to address the question of more efficient drilling; the most pertinent being as follows:
U.S. Pat. No. 2,946,565 entitled "Combination Drilling And Testing Process," discloses an apparatus that utilizes a jet nozzle directed upward to reduce pressure, allowing formation fluids to be produced while drilling. Additionally, a rubber sealing element is used to isolate hydrostatic pressure communicated by the annulus created by the tool and the borehole wall. However, unlike the present invention, the embodiments presented do not isolate the hydrostatic pressure exerted by the interior of the drillstring nor do they allow unobstructed passage of drill-cuttings. The present invention accomplishes both of these requirements (on an intermittent basis) necessary for continuous drilling under reduced pressure.
U.S. Pat. No. 2,765,146 entitled "Jetting Device For Rotary Drilling Apparatus," utilizes high velocity fluid streams directed upward to enhance the drilling process. However, unlike the present invention, the hydrostatic pressure exerted by the interior of the drillstring is not isolated. Further, the device does not reduce the area of the drillstring--borehole wall annulus to fully utilize the Bernoulli effect. As a result, hydrostatic pressure could be communicated along the sides of the upwardly directed streams. The present invention reduces this annular area (on an intermittent basis) to fully utilize the Bernoulli effect and completely isolate the hydrostatic pressure exerted by the drillstring--borehole wall annulus. Additionally, the present invention isolates the hydrostatic pressure exerted by the interior of the drillstring.
Russian Pat. No. SU197,709 entitled "Ejector Type Coring Drill," issued to Geotekhnika, although printed in Russian, from the cover sheet of the patent discloses a coring drill that has a reverse flushing capability. This apparatus would be used in a D.E.M. coring operation only and does not pertain to the present invention.
U.S. Pat. No. 4,223,747 entitled "Drilling Using Reverse Circulation", issued to L. R. Marais, discloses a drill bit apparatus whereby fluid being utilized at the near bit region is carried upward at a greater rate due to the fact that some of the fluid is pumped into the main bore of the drill string and reversed via interior effect out through the annulus between the drill string and the borehole. This apparatus does attempt to reduce the hydrostatic pressure near the bit by utilizing the Bernoulli effect in the interior of the drill string. The present invention utilizes the Bernoulli effect in the annular space between the drill string and the hole. The present invention is more effective as it operates on an intermittent basis, which allows unobstructed passage of cuttings during a half-cycle of operation.
U.S. Pat. No. 3,005,507 entitled "Fluid By-Pass For Rotary Drill Bits", issued to E. H. Clark, et al., discloses an apparatus which, does indicate a type of fluid by-pass for rotary drill bits and provides an apparatus for preventing the drill string and bit from developing a high pressure in the well bore drilling mud while the drill string and bit are being run into the well bore. The apparatus does not attempt to improve the actual drilling process.
U.S. Pat. No. 3,095,052 entitled "Reverse Circulation Sub", issued to J. G. Jackson provides for fluid communication between the inner bore of the pipe and the annulus between the drill string and the borehole via a series of portal channels. The patent discloses how the device allows more removal of quantities of fluid from around the bit by allowing more air to circulate around the bit and effect faster drilling. It is important to note that no means of isolating the fluid hydrostatic pressure is mentioned in this invention.
U.S. Pat. No. 4,049,066, entitled "Apparatus For Reducing Annular Back Pressure Near The Drill Bit," issued to Richey, has as one of its objects the removal of mud around the drill bit to order to reduce the back pressure and improve the jetting action of the mud for providing a rapid excavation and increasing the rate of penetration of the drill bit. However, it uses an annular-type turbine to reduce the pressure near the bit. The efficiency of such a device would be low and would not effect a pressure reduction as nearly as high as the Bernoulli effect.
U.S. Pat. No. 2,906,493, entitled "Washover Pipe Fluid By-Pass Sub", issued to J. W. Whitener, discloses a device for equalizing the pressure within a string of drill pipe to the pressure outside of the drill pipe as a washover pipe is lowered into the drilled borehole. As the drill string is lowered into the well, fluid entering the bottom of washover pipe is displaced upwardly and outwardly through ports to provide a pressure relief for fluid within the pipe. This apparatus does not attempt to improve the actual drilling process.